The mating type (MAT) locus of the yeast Saccharomyces cerevisiae plays a central role in determining cell type and the ability of cells to differentiate. Diploids heterozygous for the two MAT alleles (MATa/MAT alpha) are non-mating and able to carry meiosis and spore formation (gametogenesis), while diploids homozygous for MATa or MAT alpha mate but cannot sporulate. Our objectives are to understand how the two MAT alleles regulate mating type and sporulation. We intend to isolate and to study genes that are regulated not only by the nitrogen starvation conditions that induce meiosis, but also by the MAT system. Temperature-sensitive mutations in MATa will be used to determine when MAT functions are required for meiosis and temperature-sensitive sporulation (spo) mutants will be analyzed to determine their pleiotropic effects on the synthesis of sporulation proteins. We are also interested in determining in detail how MAT genes are activated, as it is evident that MAT alleles can be replaced at high frequencies by new copies transposed from other, silent chromosomal locations. We will use genetic and biochemical tests to see if MAT conversions occur via a specialized gene conversion mechanism. We will also analyze specific DNA changes that have occurred in various "stuck" and "inconvertible" MAT mutations blocked in switching by using recombinant DNA molecules transformed into yeast. We will also develop an in vitro system to the biochemistry of mating type switching. We also plan to isolate the HO gene that controls the frequency of MAT transpositions. Because HO or other genes it controls are under the control of MAT we will also try to understand how HO is regulated.